It is often desirable to characterize distribution of contaminants, such as polycyclic (or polynuclear) aromatic hydrocarbons (PAHs) or polychlorinated biphenyls (PCBs), in sediment.
Traditional sampling and analyses approaches, such as direct-push or drilling and sampling technologies, which may be relatively cheap and routine and may meet quality assurance/quality control (QA/QC) standards are more expensive and difficult when conducted on or in water environments. What is routine on dry land becomes difficult and often suffers from questionable QA/QC due to compacting of the sediments, poor recovery as a result of running sands, pudding-like slurries, etc. For example, in sand or slurry, it may be difficult to obtain a continuous core for sampling. When obtaining samples and analyzing samples, portions of the sediment are removed. These portions must be handled, cataloged and stored. Generally, several points in a site are sampled and analyzed. A map of contaminant concentration is generated based upon these points. Due to the expense of gathering sampling data, maps are frequently generated on the basis of relatively few data points.
Solid-phase extraction (SPE) techniques have been developed to, for example, analyze sediment that has been sampled or to avoid requiring sediment sampling.
One currently available SPE approach is the GERSTEL-Twister offered by GERSTEL GmbH & Co. The GERSTEL-Twister is a stir bar coated with polydimethyl siloxane (PDMS). The GERSTEL-Twister is used to stir a sediment sample. Semi-volatile organics in liquids or slurries are sorbed into the PDMS during stirring. The stir bar is rinsed and thermally desorbed, cryogenically focused and transferred to a GC column for analysis.
Another SPE approach uses an Empore disk, commercially available from 3M. Empore disks are self-contained SPE devices that adsorb PCBs from solution. The disks comprise a C-18 sorbent material held in an inert matrix (PTFE). The disks are used in a laboratory setting to isolate/capture PCBs and PAHs in sediments and fluids.
One SPE approach for passive-sampling, or analyzing sediment without requiring removing samples of sediment, is the Gore-Sorber from W.L. Gore & Associates, Inc. The Gore-Sorber comprises a continuous water impermeable membrane surrounding a sorbent material. The Gore-Sorber is delivered into the vadose zone. Volatile organic compounds (VOCs) from groundwater, soil gas, or air permeate through an outer hydrophobic membrane and are trapped on adsorbent material located inside. After a predetermined time, the Gore-Sorber is removed from its sampling location and sent to a laboratory where the VOCs are removed from selected sections (snips) of the Gore-Sorber and analyzed.
Another SPE approach for passive-sampling is the Flexible Liner Underground Technologies, Ltd. Co. (FLUTe). A specialized version of FLUTe, called the NAPL FLUTe system, is used to locate layers, filled fractures or globules of chlorinated solvent product trapped in formation. The FLUTe is placed in a punched or drilled hole where a dye in the liner reacts with the dense non-aqueous phase liquid (DNAPL) to produce a stain. When the liner is removed, the stains on the FLUTe surface indicate the location of NAPL in the hole. The FLUTe thus provides a visual indicator of presence of NAPL but does not provide more than presence/no presence capability.
Determining spatial distribution of and performing analyses on contaminants in sediments is difficult. Large-scale dredging plans may be drafted based on relatively few continuous coring and Gas Chromatography/Mass Spectrometry (GC-MS) analyses locations. This is because forming a coherent picture of contaminant distribution at a single sampling location is frequently complicated using currently available methods.
A less laborious and less expensive characterization process would enable leaving resources for remediation and treatment of the contaminated area.